A major challenge in biomedical research is to define the mechanisms for mitotic spindle assembly and how a complex array of motors and microtubule (MT) interacting proteins correctly orchestrate chromosome segregation. Defects in mitosis result in birth defects and cancer, and therefore, a full molecular understanding of mitotic mechanisms is critical for human development and health. Mitotic kinesins play essential roles in all facets of spindle function- effecting chromosome movement and segregation, cell cleavage, and regulating microtubule polymerization and depolymerization. While kinesins share common structural motifs, key amino acid changes confer unique mechanochemical properties to each kinesin which specifies its cellular function. Therefore, if we elucidate the enzymatic properties of an individual mitotic kinesin in vitro, we will gain insight into its specific role for spindle function in the complex environment of the cell where there are other molecular motors, proteins, and regulatory factors. The research proposed evaluates three representative kinesins to address questions about the mechanistic requirements for processive movement of chromosomes on the microtubule lattice, MT-MT crosslinking function for MT sliding and spindle stability, and MT shortening and MT elongation for spindle assembly and dynamics. We will use presteady-state kinetic methodologies in combination with equilibrium approaches and fluorescence microscopy to address the following three specific aims: 1) Define the Kar3Cik1 mechanochemistry for its crosslinking function of anti-parallel MTs at anaphase. 2) Define the Kar3Vik1 mechanochemistry for its accumulation at MT minus-ends to crosslink parallel MTs at the spindle poles. 3) Define the mechanistic basis of CENP-E processive stepping. PUBLIC HEALTH RELEVANCE: CENP-E, Eg5/KSP, and Kinesin-14s are essential for cell division and therefore human health and development. Their selective inhibition may lead to more effective anti-mitotic therapeutics for treatment of diseases such as cancer, symptomatic coronary artery disease, and proliferative diabetic retinopathy. The proposed studies should lead to new strategies for high throughput screens that select compounds to enhance cancer cell death rather than aneuploidy after chemotherapy. Presently, there are a number of specific chemotherapeutics targeted to Eg5/KSP and CENP-E in Phase I/II Clinical trials.